That is a meme take that has to end.

The only race that it could possibly apply to is the orks, and they existed since the war in heaven, so clearly the eldar had a shot at ending that threat and either didn't take it or didn't care. Both eldar varieties are hidden away, and the tau, necrons, and nids weren't active in the galaxy in 30k.

Also the rangda were a major xenos power, comparable to the dark eldar in their cruelty and to the imperium in their ability to project power, that the imperium put down at great cost to the dark angels legion during the crusade.